In many aircraft, main propulsion engines not only provide propulsion for the aircraft, but may also be used to drive various other rotating components such as, for example, generators, compressors, and pumps, to thereby supply electrical and/or pneumatic power. However, when an aircraft is on the ground, its main engines may not be operating. Moreover, in some instances the main propulsion engines may not be capable of supplying the power needed for propulsion as well as the power to drive these other rotating components. Thus, many aircraft include an auxiliary power unit (APU) to supplement the main propulsion engines in providing electrical and/or pneumatic power. An APU may also be used to start the propulsion engines.
An APU is typically a gas turbine engine that includes a combustion section, a power turbine section, and a compressor section. During operation of the APU, the compressor section draws in and compresses ambient air and supplies the air to the combustion section. Fuel is injected into the compressed air within the combustion section to produce the high-energy combusted air to the power turbine section. The power turbine section rotates to drive a generator for supplying electrical power, via a main shaft, and to drive its own compressor section and/or an external load compressor.
Although the aforementioned APU is generally safe and robust, certain aspects, in particular, the assembly and repair of the APU, may be improved. In one example, one or more sections of the APU may be made up of rotor groups that include one or more rotatable components (e.g., impellers and turbines) coupled to each other via a tie shaft. The tie shaft extends through bores formed through an axial length of each component and generally has a flanged end against which one component rests, a length onto which the components are mounted, and a threaded end to which a nut is mated to maintain the components on the tie shaft. During assembly or repair of the rotor group, the tie shaft may be may be repeatedly moved into and out of the bores. In some cases, if the threaded end inadvertently contacts the rotatable component, a scratch may form thereon. As a result, the useful life of the rotatable component may be reduced, which may increase a frequency of maintenance of the APU. In cases in which the scratch has dimensions that exceed an acceptable tolerance, the rotatable component may be discarded.
Accordingly, it is desirable to have a rotatable component (e.g., an impeller and/or turbine) that has an improved useful life, as compared to conventional rotatable components. In addition, it is desirable to decrease the frequency of maintenance of the APU that includes the rotatable component. Furthermore, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the inventive subject matter.